U.S. patent application number 12/605606 was filed with the patent office on 2010-02-18 for electro-optic displays.
This patent application is currently assigned to E INK CORPORATION. Invention is credited to Eva Chen, Harit Doshi, Gregg M. Duthaler, Charles Howie Honeyman, Richard D. LeCain, Simon Pang, Seungman Sohn.
Application Number | 20100039697 12/605606 |
Document ID | / |
Family ID | 34107665 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100039697 |
Kind Code |
A1 |
Honeyman; Charles Howie ; et
al. |
February 18, 2010 |
ELECTRO-OPTIC DISPLAYS
Abstract
An electro-optic display comprises first and second substrates
and a lamination adhesive layer and a layer of an electro-optic
material disposed between the first and second substrates, the
lamination adhesive layer having a thickness of from about 14 to
about 25 .mu.m.
Inventors: |
Honeyman; Charles Howie;
(Roslindale, MA) ; Doshi; Harit; (North
Chelmsford, MA) ; Sohn; Seungman; (Seodaemun, KR)
; Chen; Eva; (Somerville, MA) ; LeCain; Richard
D.; (Brighton, MA) ; Pang; Simon; (Acton,
MA) ; Duthaler; Gregg M.; (Needham, MA) |
Correspondence
Address: |
DAVID J COLE;E INK CORPORATION
733 CONCORD AVE
CAMBRIDGE
MA
02138-1002
US
|
Assignee: |
E INK CORPORATION
Cambridge
MA
|
Family ID: |
34107665 |
Appl. No.: |
12/605606 |
Filed: |
October 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11277368 |
Mar 24, 2006 |
7636191 |
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12605606 |
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10898027 |
Jul 23, 2004 |
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11277368 |
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60481133 |
Jul 24, 2003 |
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60481320 |
Sep 2, 2003 |
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Current U.S.
Class: |
359/296 |
Current CPC
Class: |
G02F 1/167 20130101;
G02F 2202/28 20130101; G02B 26/02 20130101 |
Class at
Publication: |
359/296 |
International
Class: |
G02F 1/167 20060101
G02F001/167 |
Claims
1. An electro-optic display comprising: a backplane comprising at
least one electrode; a layer of electro-optic material; and a
lamination adhesive disposed between the backplane and the layer of
electro-optic material, the lamination adhesive comprising an
adhesion promoter effective to increase the adhesion between the
lamination adhesive and the backplane.
2. An electro-optic display according to claim 1 wherein the
adhesion promoter comprises any one or more of 1-propanamine,
3-aminopropyltrimethoxysilane, 3-aminopropyldimethylethoxysilane,
and hexamethyldisilizane.
3. An electro-optic display according to claim 1 wherein the
electro-optic material comprises a particle-based electrophoretic
material comprising a suspending fluid and a plurality of
electrically charged particles suspended in the suspending fluid
and capable of moving therethrough on application of an electric
field to the electrophoretic material.
4. An electro-optic display according to claim 3 wherein the
electrophoretic material is an encapsulated electrophoretic
material in which the suspending fluid and the electrically charged
particles are encapsulated within a plurality of capsules, each of
the capsules having a capsule wall.
5. An electro-optic display according to claim 1 wherein the
electro-optic material comprises a rotating bichromal member
electro-optic material.
6. An electro-optic display according to claim 1 wherein the
electro-optic material comprises an electrochromic electro-optic
material.
7. A front plane laminate for forming an electro-optic display, the
front plane laminate being an article of manufacture comprising, in
order, a light-transmissive electrically-conductive layer, a layer
of an encapsulated electrophoretic material comprising a plurality
of capsules, each capsule comprising a capsule wall, a suspending
fluid encapsulated within the capsule wall and a plurality of
electrically charged particles suspended in the suspending fluid
and capable of moving therethrough on application of an electric
field to the electrophoretic material, and a lamination adhesive
layer, wherein the lamination adhesive layer has a peak to valley
roughness not greater than about 15 .mu.m.
8. A front plane laminate according to claim 7 wherein the
lamination adhesive layer has a peak to valley roughness not
greater than about 10 .mu.m.
9. A front plane laminate according to claim 7 wherein the
lamination adhesive layer has a peak to valley roughness not
greater than about 5 .mu.m.
10. A front plane laminate according to claim 7 wherein the front
substrate has a thickness not greater than about 20 mil.
11. A front plane laminate according to claim 10 wherein the front
substrate has a thickness not greater than about 10 mil.
12. A front plane laminate according to claim 7 further comprising
a release sheet covering the lamination adhesive.
13. A front plane laminate according to claim 12 wherein the
release sheet has a thickness not greater than about 15 mil.
14. A front plane laminate according to claim 13 wherein the
release sheet has a thickness not greater than about 10 mil.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of copending application
Ser. No. 11/277,368, filed Mar. 24, 2006 (Publication No.
2006/01762670), which is a divisional of application Ser. No.
10/898,027, filed Jul. 23, 2004 (Publication No. 2005/0122563, now
abandoned), which claims benefit of Provisional Application Ser.
No. 60/481,133, filed Jul. 24, 2003 and Ser. No. 60/481,320, filed
Sep. 2, 2003.
[0002] This application is also related to application Ser. No.
10/064,389, filed Jul. 2, 2002 (Publication No. 2003/0025855, now
U.S. Pat. No. 6,831,769), which itself claims priority of
Provisional Application Ser. No. 60/304,117, filed Jul. 9,
2001.
[0003] The entire contents of these applications, and of all other
U.S. patents and published and copending applications mentioned
below, are herein incorporated by reference.
BACKGROUND OF INVENTION
[0004] This invention relates to improvements in electro-optic
displays. More specifically, in one aspect this invention relates
to electro-optic media and displays in which the thickness of a
lamination adhesive layer is controlled to avoid certain problems
otherwise experienced in such displays. In another aspect, this
invention relates to the prevention of void growth in electro-optic
displays.
[0005] Electro-optic displays comprise a layer of electro-optic
material, a term which is used herein in its conventional meaning
in the imaging art to refer to a material having first and second
display states differing in at least one optical property, the
material being changed from its first to its second display state
by application of an electric field to the material. Although the
optical property is typically color perceptible to the human eye,
it may be another optical property, such as optical transmission,
reflectance, luminescence or, in the case of displays intended for
machine reading, pseudo-color in the sense of a change in
reflectance of electromagnetic wavelengths outside the visible
range.
[0006] The terms "bistable" and "bistability" are used herein in
their conventional meaning in the imaging art to refer to displays
comprising display elements having first and second display states
differing in at least one optical property, and such that after any
given element has been driven, by means of an addressing pulse of
finite duration, to assume either its first or second display
state, after the addressing pulse has terminated, that state will
persist for at least several times, for example at least four
times, the minimum duration of the addressing pulse required to
change the state of the display element. It is shown in published
U.S. Patent Application No. 2002/0180687 that some particle-based
electrophoretic displays capable of gray scale are stable not only
in their extreme black and white states but also in their
intermediate gray states, and the same is true of some other types
of electro-optic displays. This type of display is properly called
"multi-stable" rather than bistable, although for convenience the
term "bistable" may be used herein to cover both bistable and
multi-stable displays.
[0007] Several types of electro-optic displays are known. One type
of electro-optic display is a rotating bichromal member type as
described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782;
5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467;
and 6,147,791 (although this type of display is often referred to
as a "rotating bichromal ball" display, the term "rotating
bichromal member" is preferred as more accurate since in some of
the patents mentioned above the rotating members are not
spherical). Such a display uses a large number of small bodies
(typically spherical or cylindrical) which have two or more
sections with differing optical characteristics, and an internal
dipole. These bodies are suspended within liquid-filled vacuoles
within a matrix, the vacuoles being filled with liquid so that the
bodies are free to rotate. The appearance of the display is changed
to applying an electric field thereto, thus rotating the bodies to
various positions and varying which of the sections of the bodies
is seen through a viewing surface.
[0008] Another type of electro-optic display uses an electrochromic
medium, for example an electrochromic medium in the form of a
nanochromic film comprising an electrode formed at least in part
from a semi-conducting metal oxide and a plurality of dye molecules
capable of reversible color change attached to the electrode; see,
for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood,
D., Information Display, 18(3), 24 (March 2002). See also Bach, U.,
et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this
type are also described, for example, in U.S. Pat. No. 6,301,038,
International Application Publication No. WO 01/27690, and in U.S.
Patent Application 2003/0214695. This type of medium is also
typically bistable.
[0009] Another type of electro-optic display, which has been the
subject of intense research and development for a number of years,
is the particle-based electrophoretic display, in which a plurality
of charged particles move through a suspending fluid under the
influence of an electric field. Electrophoretic displays can have
attributes of good brightness and contrast, wide viewing angles,
state bistability, and low power consumption when compared with
liquid crystal displays. Nevertheless, problems with the long-term
image quality of these displays have prevented their widespread
usage. For example, particles that make up electrophoretic displays
tend to settle, resulting in inadequate service-life for these
displays.
[0010] Numerous patents and applications assigned to or in the
names of the Massachusetts Institute of Technology (MIT) and E Ink
Corporation have recently been published describing encapsulated
electrophoretic media. Such encapsulated media comprise numerous
small capsules, each of which itself comprises an internal phase
containing electrophoretically-mobile particles suspended in a
liquid suspending medium, and a capsule wall surrounding the
internal phase. Typically, the capsules are themselves held within
a polymeric binder to form a coherent layer positioned between two
electrodes. Encapsulated media of this type are described, for
example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;
6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;
6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;
6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;
6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;
6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;
6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;
6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;
6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;
6,704,133; 6,710,540; 6,721,083; 6,724,519; and 6,727,881; and U.S.
Patent Applications Publication Nos. 2002/0019081; 2002/0021270;
2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677;
2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832;
2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687;
2002/0180688; 2002/0185378; 2003/0011560; 2003/0011868;
2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755;
2003/0053189; 2003/0102858; 2003/0132908; 2003/0137521;
2003/0137717; 2003/0151702; 2003/0189749; 2003/0214695;
2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839;
2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422;
2004/0105036; and 2004/0112750; and International Applications
Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO
00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO
01/08241; WO 03/092077; WO 03/107315; and WO 2004/049045.
[0011] Many of the aforementioned patents and applications
recognize that the walls surrounding the discrete microcapsules in
an encapsulated electrophoretic medium could be replaced by a
continuous phase, thus producing a so-called "polymer-dispersed
electrophoretic display" in which the electrophoretic medium
comprises a plurality of discrete droplets of an electrophoretic
fluid and a continuous phase of a polymeric material, and that the
discrete droplets of electrophoretic fluid within such a
polymer-dispersed electrophoretic display may be regarded as
capsules or microcapsules even though no discrete capsule membrane
is associated with each individual droplet; see for example, the
aforementioned 2002/0131147. Accordingly, for purposes of the
present application, such polymer-dispersed electrophoretic media
are regarded as sub-species of encapsulated electrophoretic
media.
[0012] An encapsulated electrophoretic display typically does not
suffer from the clustering and settling failure mode of traditional
electrophoretic devices and provides further advantages, such as
the ability to print or coat the display on a wide variety of
flexible and rigid substrates. (Use of the word "printing" is
intended to include all forms of printing and coating, including,
but without limitation: pre-metered coatings such as patch die
coating, slot or extrusion coating, slide or cascade coating,
curtain coating; roll coating such as knife over roll coating,
forward and reverse roll coating; gravure coating; dip coating;
spray coating; meniscus coating; spin coating; brush coating; air
knife coating; silk screen printing processes; electrostatic
printing processes; thermal printing processes; ink jet printing
processes; and other similar techniques.) Thus, the resulting
display can be flexible. Further, because the display medium can be
printed (using a variety of methods), the display itself can be
made inexpensively.
[0013] Known encapsulated electrophoretic displays can be divided
into two main types, referred to hereinafter for convenience as
"single particle" and "dual particle" respectively. A single
particle medium has only a single type of electrophoretic particle
suspended in a colored medium, at least one optical characteristic
of which differs from that of the particles. (In referring to a
single type of particle, we do not imply that all particles of the
type are absolutely identical. For example, provided that all
particles of the type possess substantially the same optical
characteristic and a charge of the same polarity, considerable
variation in parameters such as particle size and electrophoretic
mobility can be tolerated without affecting the utility of the
medium.) The optical characteristic is typically color visible to
the human eye, but may, alternatively or in addition, be any one or
more of reflectivity, retroreflectivity, luminescence,
fluorescence, phosphorescence, or color in the broader sense of
meaning a difference in absorption or reflectance at non-visible
wavelengths. When such a medium is placed between a pair of
electrodes, at least one of which is transparent, depending upon
the relative potentials of the two electrodes, the medium can
display the optical characteristic of the particles (when the
particles are adjacent the electrode closer to the observer,
hereinafter called the "front" electrode) or the optical
characteristic of the suspending medium (when the particles are
adjacent the electrode remote from the observer, hereinafter called
the "rear" electrode, so that the particles are hidden by the
colored suspending medium).
[0014] A dual particle medium has two different types of particles
differing in at least one optical characteristic and a suspending
fluid which may be uncolored or colored, but which is typically
uncolored. The two types of particles differ in electrophoretic
mobility; this difference in mobility may be in polarity (this type
may hereinafter be referred to as an "opposite charge dual
particle" medium) and/or magnitude. When such a dual particle
medium is placed between the aforementioned pair of electrodes,
depending upon the relative potentials of the two electrodes, the
medium can display the optical characteristic of either set of
particles, although the exact manner in which this is achieved
differs depending upon whether the difference in mobility is in
polarity or only in magnitude. For ease of illustration, consider
an electrophoretic medium in which one type of particles are black
and the other type white. If the two types of particles differ in
polarity (if, for example, the black particles are positively
charged and the white particles negatively charged), the particles
will be attracted to the two different electrodes, so that if, for
example, the front electrode is negative relative to the rear
electrode, the black particles will be attracted to the front
electrode and the white particles to the rear electrode, so that
the medium will appear black to the observer. Conversely, if the
front electrode is positive relative to the rear electrode, the
white particles will be attracted to the front electrode and the
black particles to the rear electrode, so that the medium will
appear white to the observer.
[0015] If the two types of particles have charges of the same
polarity, but differ in electrophoretic mobility (this type of
medium may hereinafter to referred to as a "same polarity dual
particle" medium), both types of particles will be attracted to the
same electrode, but one type will reach the electrode before the
other, so that the type facing the observer differs depending upon
the electrode to which the particles are attracted. For example
suppose the previous illustration is modified so that both the
black and white particles are positively charged, but the black
particles have the higher electrophoretic mobility. If now the
front electrode is negative relative to the rear electrode, both
the black and white particles will be attracted to the front
electrode, but the black particles, because of their higher
mobility, will reach it first, so that a layer of black particles
will coat the front electrode and the medium will appear black to
the observer. Conversely, if the front electrode is positive
relative to the rear electrode, both the black and white particles
will be attracted to the rear electrode, but the black particles,
because of their higher mobility will reach it first, so that a
layer of black particles will coat the rear electrode, leaving a
layer of white particles remote from the rear electrode and facing
the observer, so that the medium will appear white to the observer:
note that this type of dual particle medium requires that the
suspending fluid to sufficiently transparent to allow the layer of
white particles remote from the rear electrode to be readily
visible to the observer. Typically, the suspending fluid in such a
display is not colored at all, but some color may be incorporated
for the purpose of correcting any undesirable tint in the white
particles seen therethrough.
[0016] Certain of the aforementioned E Ink and MIT patents and
applications describe electrophoretic media which have more than
two types of electrophoretic particles within a single capsule. For
present purposes, such multi-particle media are regarded as a
sub-class of dual particle media.
[0017] Both single and dual particle electrophoretic displays may
be capable of intermediate gray states having optical
characteristics intermediate the two extreme optical states already
described.
[0018] A related type of electrophoretic display is a so-called
"microcell electrophoretic display". In a microcell electrophoretic
display, the charged particles and the suspending fluid are not
encapsulated within capsules but instead are retained within a
plurality of cavities formed within a carrier medium, typically a
polymeric film. See, for example, International Application
Publication No. WO 02/01281, and U.S. Patent Application
Publication No. 2002/0075556, both assigned to Sipix Imaging,
Inc.
[0019] Although electrophoretic media are often opaque (since, for
example, in many electrophoretic media, the particles substantially
block transmission of visible light through the display) and
operate in a reflective mode, many electrophoretic displays can be
made to operate in a so-called "shutter mode" in which one display
state is substantially opaque and one is light-transmissive. See,
for example, the aforementioned U.S. Pat. Nos. 6,130,774 and
6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823;
6,225,971; and 6,184,856. Dielectrophoretic displays, which are
similar to electrophoretic displays but rely upon variations in
electric field strength, can operate in a similar mode; see U.S.
Pat. No. 4,418,346. Other types of electro-optic displays may also
be capable of operating in shutter mode.
[0020] An electro-optic display normally comprises a layer of
electro-optic material and at least two other layers disposed on
opposed sides of the electro-optic material, one of these two
layers being an electrode layer. In most such displays both the
layers are electrode layers, and one or both of the electrode
layers are patterned to define the pixels of the display. For
example, one electrode layer may be patterned into elongate row
electrodes and the other into elongate column electrodes running at
right angles to the row electrodes, the pixels being defined by the
intersections of the row and column electrodes. Alternatively, and
more commonly, one electrode layer has the form of a single
continuous electrode and the other electrode layer is patterned
into a matrix of pixel electrodes, each of which defines one pixel
of the display. In another type of electro-optic display, which is
intended for use with a stylus, print head or similar movable
electrode separate from the display, only one of the layers
adjacent the electro-optic layer comprises an electrode, the layer
on the opposed side of the electro-optic layer typically being a
protective layer intended to prevent the movable electrode damaging
the electro-optic layer.
[0021] The manufacture of a three-layer electro-optic display
normally involves at least one lamination operation. For example,
in several of the aforementioned MIT and E Ink patents and
applications, there is described a process for manufacturing an
encapsulated electrophoretic display in which an encapsulated
electrophoretic medium comprising capsules in a binder is coated on
to a flexible substrate comprising indium-tin-oxide or a similar
conductive coating (which acts as an one electrode of the final
display) on a plastic film, the capsules/binder coating being dried
to form a coherent layer of the electrophoretic medium firmly
adhered to the substrate. Separately, a backplane, containing an
array of pixel electrodes and an appropriate arrangement of
conductors to connect the pixel electrodes to drive circuitry, is
prepared. To form the final display, the substrate having the
capsule/binder layer thereon is laminated to the backplane using a
lamination adhesive. (A very similar process can be used to prepare
an electrophoretic display useable with a stylus or similar movable
electrode by replacing the backplane with a simple protective
layer, such as a plastic film, over which the stylus or other
movable electrode can slide.) In one preferred form of such a
process, the backplane is itself flexible and is prepared by
printing the pixel electrodes and conductors on a plastic film or
other flexible substrate. It will readily be apparent to those
skilled in the manufacture of electro-optic displays that other
types of electro-optic media, for example microcell electrophoretic
media and rotating bichromal member media, can be laminated in an
exactly analogous manner. The obvious lamination technique for mass
production of displays by this process is roll lamination using a
lamination adhesive. In a modified form of such a lamination
process, as described for example in the aforementioned
2004/0027327, the substrate having the capsule/binder layer thereon
is first laminated to a layer of adhesive carried on a release
sheet to form a structure called a "front plane laminate" (or
"FPL"), and thereafter the release sheet is peeled from the front
plane laminate and the remaining layers of the front plane laminate
are laminated to a backplane using the adhesive layer exposed by
removal of the release sheet, thereby forming the final display. A
related lamination process using two separate laminations is
described in copending application Ser. No. 10/605,024, filed Sep.
2, 2003 (Publication No. 2004/0155857).
[0022] In practice, these lamination processes impose stringent
requirements upon both the mechanical and electrical properties of
the lamination adhesive. In the final electro-optic display, the
lamination adhesive is located between the electrodes which apply
the electric field needed to change the electrical state of the
electro-optic medium, so that the electrical properties of the
adhesive become crucial. As will be apparent to electrical
engineers, the volume resistivity of the lamination adhesive
becomes important, since the voltage drop across the electro-optic
medium is essentially equal to the voltage drop across the
electrodes, minus the voltage drop across the lamination adhesive.
If the resistivity of the adhesive layer is too high, a substantial
voltage drop will occur within the adhesive layer, requiring an
increase in voltage across the electrodes. Increasing the voltage
across the electrodes in this manner is undesirable, since it
increases the power consumption of the display, and may require the
use of more complex and expensive control circuitry to handle the
increased voltage involved. Hence, it has hitherto been assumed
that it is desirable to make the adhesive layer as thin as
possible, consistent with satisfactory adhesion, in order to
minimize the "wasted" voltage drop across the adhesive layer.
[0023] There are numerous other electrical and mechanical
constraints which a lamination adhesive used in an electro-optic
display must satisfy, as discussed in detail in the aforementioned
2003/0025855. One particular problem not there discussed, but to
which the present invention relates, is the "void problem". To
ensure a high quality display, it is essential that the final
display be free from voids, since such voids produce visible
defects in images written on the display. To ensure that the final
display is free from voids, it is essential that both the
lamination to form the front plane laminate (when effected) and the
final lamination to the backplane be carried out without the
formation of voids. It is also necessary that the final display be
able to withstand substantial temperature changes (such as may
occur, for example, when a portable computer or personal digital
assistant is removed from an air-conditioned car to outdoor sun on
a hot day) without inducing or aggravating the formation of voids,
since it has been found that some displays, which initially appear
essentially free from voids, can develop objectionable voids when
exposed to such temperature changes. This phenomenon may be termed
"void re-growth".
[0024] It has now been found that, especially in encapsulated
electrophoretic displays, such void re-growth is a function of the
thickness of the lamination adhesive, and that to avoid the
formation of objectionable voids the thickness of the lamination
adhesive should not be reduced below a critical value. This value
is believed to be somewhat dependent upon the specific material,
typically a polymer, used as the lamination adhesive, the exact
type of electro-optic medium used and the lamination conditions
employed, but is typically around 14 .mu.m.
[0025] Thus, in one aspect this invention provides an electro-optic
display having a controlled thickness of lamination adhesive.
[0026] Also, from the foregoing discussion, it will be seen that
many electro-optic displays are of a "hybrid" type and consist of
an asymmetric stack of materials with highly dissimilar properties.
For example, the aforementioned front plane laminate ("FPL")
comprises, in order, a polymeric film substrate, a
light-transmissive electrode, a layer of a electro-optic medium, a
layer of lamination adhesive and a release sheet; to produce the
final display, this front plane laminate is laminated in the manner
already described to a backplane comprising a plurality of pixel
electrodes on a glass or other substrate. The release sheet of the
front plane laminate is removed prior to its lamination to the
backplane, so that the final structure comprises, in order, the
polymeric film substrate, light-transmissive electrode, layer of a
electro-optic medium, layer of lamination adhesive, pixel
electrodes and glass or other substrate. Such a structure is in
principle structurally unstable, as differences in thermal and
moisture expansion coefficients of the polymeric film and glass
substrates induce very large stress and strains in the display.
Under particular stresses, especially those realized at elevated
temperatures, the instability manifests itself in the formation of
voids (i.e. delamination, hereafter called void growth of the
layers derived from the front plane laminate from the glass
substrate).
[0027] This invention provides approaches to eliminating, or at
least reducing, void growth in electro-optic displays.
SUMMARY OF INVENTION
[0028] Accordingly, in one aspect, this invention provides an
electro-optic display comprising first and second substrates and a
lamination adhesive layer and a layer of an electro-optic material
disposed between the first and second substrates, the lamination
adhesive layer having a thickness of from about 14 to about 25
.mu.m.
[0029] This electro-optic display may hereinafter for convenience
be referred to as the "controlled lamination adhesive thickness" or
"CLAT" display of the present invention.
[0030] In such a CLAT display, the lamination adhesive layer
desirably has a thickness of from about 14 to about 20 .mu.m. In
one form of the CLAT display, the electro-optic layer comprises a
particle-based electrophoretic material comprising a suspending
fluid and a plurality of electrically charged particles suspended
in the suspending fluid and capable of moving therethrough on
application of an electric field to the electrophoretic material.
Such an electrophoretic material may be an encapsulated
electrophoretic material in which the suspending fluid and the
electrically charged particles and encapsulated within a plurality
of capsules, each of the capsules having a capsule wall. (In this
case, and in the other cases where encapsulated electrophoretic
material is mentioned below, the encapsulated electrophoretic
material may be of the polymer-dispersed type with the suspending
fluid and the electrically charged particles in the form of a
plurality of droplets dispersed in a continuous phase which in
effect acts as capsule walls for the droplets, although no discrete
capsule wall for each droplet is present.)
[0031] The lamination adhesives used in CLAT displays may be
similar to those used in the aforementioned 2003/0025855. Thus, the
lamination adhesive may have one or more of the following
characteristics:
[0032] (a) a volume resistivity, measured at 10.degree. C., which
does not change by a factor of more than about 3 after being at
25.degree. C. and 45 percent relative humidity for 1000 hours;
[0033] (b) a peel strength from an electrode material in contact
with the lamination adhesive of at least about 2 lb/inch (about 35
Newtons m.sup.-1);
[0034] (c) a volume resistivity which changes by a factor of less
than about 10 within a range of 10 to 90 percent relative humidity
and over a temperature range of 10 to 50.degree. C.;
[0035] (d) a shear modulus at 120.degree. C. of not more than about
1 megaPascal;
[0036] (e) in the case of displays using an electrophoretic
material, a dielectric constant and volume resistivity such that
the product of the dielectric constant and the volume resistivity
of the lamination adhesive are not greater than the product of the
dielectric constant and the volume resistivity of the
electrophoretic material within a range of 10 to 90 percent
relative humidity and over a temperature range of 10 to 50.degree.
C.; and
[0037] (f) at least one of an ultra-violet stabilizer and a light
absorbing material incorporated in the lamination adhesive.
[0038] This invention also provides a front plane laminate for
forming an electro-optic display, the front plane laminate being an
article of manufacture comprising, in order, a light-transmissive
electrically-conductive layer, a layer of an electro-optic
material, a lamination adhesive having a thickness of from about 14
to about 25 .mu.m, and a release sheet.
[0039] In such a front plane laminate, the lamination adhesive
layer desirably has a thickness of from about 14 to about 20 .mu.m.
In one form of the front plane lamination, the electro-optic layer
may comprise a particle-based electrophoretic material comprising a
suspending fluid and a plurality of electrically charged particles
suspended in the suspending fluid and capable of moving
therethrough on application of an electric field to the
electrophoretic material. Such an electrophoretic material may be
an encapsulated electrophoretic material in which the suspending
fluid and the electrically charged particles and encapsulated
within a plurality of capsules, each of the capsules having a
capsule wall.
[0040] This invention also provides an electro-optic display
comprising: a backplane comprising at least one electrode; a layer
of electro-optic material; and a lamination adhesive disposed
between the backplane and the layer of electro-optic material, the
lamination adhesive comprising an adhesion promoter effective to
increase the adhesion between the lamination adhesive and the
backplane.
[0041] In such an electro-optic display, the adhesion promoter may
comprise any one or more of 1-propanamine,
3-aminopropyltrimethoxysilane, 3-aminopropyldimethylethoxysilane,
and hexamethyldisilizane.
[0042] This invention also provides a front plane laminate for
forming an electro-optic display, the front plane laminate being an
article of manufacture comprising, in order, a front substrate, a
light-transmissive electrically-conductive layer, a layer of an
electro-optic material, and a lamination adhesive layer, wherein
the front substrate has a thickness not greater than about 20 mil
(about 0.5 mm).
[0043] In such a front plane laminate, the front substrate
desirably has a thickness not greater than about 10 mil (about 0.25
mm). The front plane laminate may further comprise a release sheet
covering the lamination adhesive. Such a release sheet desirably
has a thickness not greater than about 15 mil (about 0.37 mm), and
preferably not greater than about 10 mil (about 0.25 mm). The
electro-optic material may be an encapsulated electrophoretic
material comprising a plurality of capsules, each capsule
comprising a capsule wall, a suspending fluid encapsulated within
the capsule wall and a plurality of electrically charged particles
suspended in the suspending fluid and capable of moving
therethrough on application of an electric field to the
electrophoretic material.
[0044] This invention also provides a front plane laminate for
forming an electro-optic display, the front plane laminate being an
article of manufacture comprising, in order, a light-transmissive
electrically-conductive layer, a layer of an electro-optic
material, a lamination adhesive layer, and a release sheet wherein
the release sheet has a thickness not greater than about 15 mil
(about 0.37 mm).
[0045] In such a front plane laminate the release sheet desirably
has a thickness not greater than about 10 mil (about 0.25 mm). The
electro-optic material may be an encapsulated electrophoretic
material comprising a plurality of capsules, each capsule
comprising a capsule wall, a suspending fluid encapsulated within
the capsule wall and a plurality of electrically charged particles
suspended in the suspending fluid and capable of moving
therethrough on application of an electric field to the
electrophoretic material.
[0046] This invention also provides a front plane laminate for
forming an electro-optic display, the front plane laminate being an
article of manufacture comprising, in order, a light-transmissive
electrically-conductive layer, a layer of an encapsulated
electrophoretic material comprising a plurality of capsules, each
capsule comprising a capsule wall, a suspending fluid encapsulated
within the capsule wall and a plurality of electrically charged
particles suspended in the suspending fluid and capable of moving
therethrough on application of an electric field to the
electrophoretic material, and a lamination adhesive layer, wherein
the lamination adhesive layer has a peak to valley roughness not
greater than about 15 .mu.m.
[0047] In such a front plane laminate, the lamination adhesive
layer desirably has a peak to valley roughness not greater than
about 10 .mu.m, preferably not greater than about 5 .mu.m.
[0048] Finally, this invention provides a front plane laminate for
forming an electro-optic display, the front plane laminate being an
article of manufacture comprising, in order, a light-transmissive
electrically-conductive layer, a layer of an encapsulated
electrophoretic material comprising a plurality of capsules, each
capsule comprising a capsule wall, a suspending fluid encapsulated
within the capsule wall and a plurality of electrically charged
particles suspended in the suspending fluid and capable of moving
therethrough on application of an electric field to the
electrophoretic material, and a lamination adhesive layer, wherein
the lamination adhesive layer has local surface angles not greater
than about 15.degree. from the horizontal.
[0049] In such a front plane laminate, the lamination adhesive
layer desirably has local surface angles not greater than about
10.degree. from the horizontal, preferably not greater than about
5.degree. from the horizontal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1A is a schematic side elevation of a electro-optic
display as it is formed at room temperature.
[0051] FIG. 1B is a schematic side elevation similar to that of
FIG. 1A but taken at an elevated temperature and showing the
buckling of the front substrate of the electro-optic display.
[0052] FIG. 2 is a schematic side elevation of a front plane
laminate of the present invention showing the uneven surfaces of
the electro-optic material and lamination adhesive layer.
[0053] FIG. 3 is a schematic side elevation similar to that of FIG.
2 but prior to the application of the lamination adhesive and shows
the manner in which the peak-to-valley roughness and the local
surface angles of the electro-optic layer are measured.
[0054] FIG. 4A is a photomicrograph of a front plane laminate
having a peak-to-valley roughness greater than 5 .mu.m.
[0055] FIG. 4B is a photomicrograph showing the voids which result
from the lamination of the front plane laminate of FIG. 4A to a
glass backplane.
DETAILED DESCRIPTION
[0056] In the controlled lamination adhesive thickness display of
the present invention, and in the corresponding front plane
laminate, the thickness of the lamination adhesive is controlled to
eliminate, or at least substantially reduce, void re-growth in the
final display. Empirically, it has been found that typically a
minimum lamination adhesive thickness of about 14 .mu.m is needed
to reduce void re-growth to an acceptable level, and that
increasing the lamination adhesive thickness further, up to about
18 .mu.m, may provide additional control of void re-growth.
Increasing the lamination adhesive thickness above about 18 .mu.m
does not appear to increase resistance to void re-growth but, as
already noted, it is desirable to avoid an unnecessarily thick
lamination adhesive layer in order to avoid a large voltage drop
across this layer, so in practice it is advantageous to limit the
lamination adhesive thickness to not more than about 25, or
preferably 20, .mu.m.
[0057] Apart from controlling the thickness of the lamination
adhesive in accordance with the present invention, the lamination
adhesive may be as described in the aforementioned 2003/0025855;
thus, the lamination adhesive may have any one of more of the
following properties:
[0058] (a) a volume resistivity, measured at 10.degree. C., which
does not change by a factor of more than about 3 after being at
25.degree. C. and 45 percent relative humidity for 1000 hours;
[0059] (b) a peel strength from an electrode material in contact
with the lamination adhesive of at least about 2 lb/inch (about 35
Newtons m.sup.-1);
[0060] (c) a volume resistivity which changes by a factor of less
than about 10 within a range of 10 to 90 percent relative humidity
and over a temperature range of 10 to 50.degree. C.;
[0061] (d) a shear modulus at 120.degree. C. of not more than about
1 megaPascal;
[0062] (e) in the case of CLAT displays using an electrophoretic
material, a product of the dielectric constant and the volume
resistivity of the lamination adhesive not greater than the product
of the dielectric constant and the volume resistivity of the
electrophoretic material within a range of 10 to 90 percent
relative humidity and over a temperature range of 10 to 50.degree.
C.;
[0063] (f) an ultra-violet stabilizer incorporated into the
lamination adhesive; and
[0064] (g) a light absorbing material incorporated into the
lamination adhesive.
[0065] The CLAT display and front plane laminate of the invention,
are especially, but not exclusively, intended to use with
encapsulated electrophoretic media; as discussed above, such media
comprise a plurality of capsules, each capsule comprising a capsule
wall, a suspending fluid encapsulated within the capsule wall and a
plurality of electrically charged particles suspended in the
suspending fluid and capable of moving therethrough on application
of an electric field to the electrophoretic material. Other types
of electro-optic media, for example microcell electrophoretic,
rotating bichromal member media, and electrochromic media, may also
be used.
[0066] The following Example 1 is now given, though by way of
illustration only, to show details of preferred materials and
techniques for use in CLAT display of the present invention.
Example 1
[0067] An internal phase was prepared comprising polymer-coated
titania white particles and polymer-coated carbon black particles
in a hydrocarbon suspending fluid. This internal phase was
encapsulated in gelatin/acacia microcapsules substantially as
described in Paragraphs [0069] to [0074] of the aforementioned
2002/0180687. The resultant microcapsules were separated by size
and capsules having an average particle size of about 35 .mu.m were
used in the following experiments. The microcapsules were mixed
into a slurry with a polyurethane binder and coated by a
roll-to-roll process at a dry coating weight of 18 g m.sup.-2 on to
the surface of a 7 mil (177 .mu.m) poly(ethylene terephthalate)
film carrying an indium tin oxide (ITO) layer on one surface, the
microcapsules being deposited on the ITO-covered surface,
substantially as described in Paragraphs [0075] and [0076] of the
aforementioned 2002/0180687. The capsule-bearing film was then
formed into a front plane laminate by laminating it to a layer of a
polyurethane lamination adhesive carried on a release sheet, this
lamination being effected at 65 psig (0.51 mPa) at a speed of 6
inches/min (2.5 mm/sec) using a Western Magnum twin roll Laminator
with both rolls held at 120.degree. C. The thickness of the
lamination adhesive on the release sheet was varied to vary the
thickness of the corresponding layer in the final display. To
provide experimental single-pixel displays suitable for use in
these experiments, pieces of the resultant front plane laminate had
their release sheets removed and were then laminated at 75.degree.
C. to the ITO-covered surfaces of ITO-on-glass substrates.
[0068] To test void re-growth, the displays thus produced were
subjected to a temperature of 90.degree. C. and a relative humidity
of 17 percent for 17 hours, and the percentage of voids in the
display was measured before exposure to the elevated temperature,
after 5 hours exposure and after the 17 hours exposure. Eight
sections of each display was measured and the results reported
below are the average of the eight sections. The panels were
switched to their white optical state, and a digital image of each
section was taken. All pixels of the digital image were analyzed to
determine whether they were above or below a threshold (and thus
white or dark) and the total area of white and dark pixels was
calculated, and hence the percentage void area, the black pixels
being assumed to represent void area.
[0069] The thickness of the lamination adhesive layer initially
formed on the release sheet was adjusted to provide nominal
adhesive thicknesses of 12, 14, 16 and 16 .mu.m in the final
displays. The actual thicknesses measured are as shown in Table 1
below:
TABLE-US-00001 TABLE 1 Target Thickness Measured thickness Standard
Deviation (.mu.m) (.mu.m) (.mu.m) 12 11.7 0.8 14 13.6 0.9 16 15.6
1.3 18 18.1 1.5
[0070] The results obtained are shown in Table 2 below; in the
third and fourth columns, the figures given are the increase in
voids at 5 hours and 17 hours respectively, compared with the
initial figures for the same samples, while the last column gives
the total percentage of voids at the end of the experiment:
TABLE-US-00002 TABLE 2 Target Thickness Initial % 5 Hour .DELTA. %
Final .DELTA. % (.mu.m) void void void Total % void 12 0.06 0.39
0.68 0.74 14 0.02 0.08 0.13 0.14 16 0.03 0.01 0.09 0.12 18 0.09
-0.04 0.06 0.15
[0071] From Table 2, it will be seen that the display having the 12
.mu.m lamination adhesive layer suffered substantial void re-growth
during the experiment, but the extent of void re-growth was much
less in the other displays.
[0072] The aspects of the present invention other than the CLAT
display and front plane laminate will now be discussed. As already
indicated, these other aspects of the invention relate mainly to
controlling the thicknesses and mechanical properties of the
various layers of an electro-optic display or front plane
laminate.
[0073] Before discussing these aspects of the present invention in
detail, it is deemed advisable to first consider the forces which
lead to void growth and related defects in electro-optic displays.
Void growth in electro-optic displays arises from a combination of
factors. As already noted, the structural mechanics of this type of
display suggest that the display will tend to be unstable at
elevated temperatures. When, as is typically the case, an
electro-optic display is constructed with a front substrate
comprising a polymeric film (through which an observer views the
display) and a glass or similar rigid backplane, differences in
coefficient of thermal expansion (CTE) and coefficient of relative
humidity expansion (CHE) between the materials used in the
construction create stresses and strains large enough to cause
curling or warping of the display. More specifically, the CTE of
the polymeric film will typically be substantially greater than
that of the rigid backplane. Thus, even though as originally formed
at ambient temperature, the front substrate lies parallel to the
backplane, as illustrated in FIG. 1A of the accompanying drawings
(the electro-optic medium is omitted from FIGS. 1A and 1B for ease
of illustration), under extreme high temperature (and/or high
humidity) conditions, the polymeric film substrate enters into a
state of compression because its bond to the rigid glass substrate
prevents the polymeric from expanding as much as it desires. Under
the action of this compressive stress, the polymeric film
buckles.
[0074] It is difficult to conceive of ways to prevent the emergence
of compressive stresses in this high temperature/relative humidity
environmental limit, given the structural elements in this system.
One cannot simply build the display panel at elevated temperatures
so that only minimal temperature gradients are experienced during
environmental stressing, as such a panel will dramatically curl
when lowered to room temperature, as the polymeric film shrinks
substantially more with temperature than the backplane. Moreover,
if this approach were used, the display might experience
catastrophic failure such as edge seal delamination when it is
stressed in cold temperature extremes, such delamination being
caused by tension generated in the front substrate during
cooling.
[0075] One can look to structural theory to gain insight into what
parameters to modify in order to reduce the tendency of the film to
delaminate. However, one finds that the critical buckling force
P.sub.crit per unit width may be predicted to a first order using
the following relationship (the one-dimensional case is considered
for simplicity):
P crit ~ ( kEt 3 3 ) 1 / 2 ##EQU00001##
[0076] where k is the tensile stiffness of the adhesive connecting
the electro-optic material layer to the backplane, E is the Young's
modulus of the polymeric front substrate, and t is the thickness of
this front substrate. From this simple relationship, it can be seen
that it is desirable to increase the tensile stiffness of the
adhesive, the stiffness of the bond between the adhesive and the
backplane and the stiffness of the bond between the adhesive and
the electro-optic layer.
[0077] Increasing the stiffness of the adhesive can be troublesome,
however, as a stiffer adhesive tends to be much more difficult to
laminate using thermal lamination processes, such as those
described in the aforementioned 2004/0027327. It is instead
desirable to influence the strength of the bonds between the
lamination adhesive layer, the backplane and the electro-optic
layer. To strengthen the bond between the lamination adhesive and
the backplane, one can use adhesion promoters such as
1-propanamine, 3-(trimethoxysilyl) (more systematically named
3-amino-propyltrimethoxysilane), 3-aminopropyldimethylethoxysilane,
hexamethyldisilizane or other such materials. Other materials that
form covalent bonds to the glass surface (or coatings on the glass
surface) and have suitable chemistry for bonding to the lamination
adhesive may also be used. To strengthen the bond with the
electro-optic layer, it is primarily desirable to ensure that the
adhesive bonds well to the materials which forms the surface of
this layer adjacent the lamination adhesive; in the case of
encapsulated electrophoretic media, this material is typically the
polymeric binder used to form the capsules into a coherent layer,
as described in the aforementioned E Ink and MIT patents and
applications.
[0078] Alternatively or in addition, it may be desirable to use
state-of-the-art cleaning methods to prepare the surface of the
backplane. Ultra-violet, ozone, plasma, solvent cleaning, and other
such methods known to those skilled in the art and may be used for
this purpose. Corona discharge processing of the adhesive is
another surface preparation method that may prove useful.
[0079] At first glance, the above equation also suggests that it is
desirable to increase E and t of the polymeric front substrate.
However, further analysis of the stress/strain state of a hybrid
display show that increasing E and t increases the loading on the
polymeric substrate in absolute terms, so increases in these
parameters tend to be counter-productive.
[0080] The above equation is, however, based upon a simplified
model of the display, and practical experience indicates that there
are additional factors which affect the void growth process in
encapsulated electrophoretic and other electro-optic displays.
Specifically, the experience of the present inventors and their
co-workers has made it clear that, when a display is formed by
laminating a front plane laminate to a backplane in the manner
already described, the surface roughness of the front plane
laminate should be kept small and the lamination adhesive thickness
relatively large to ensure that a high quality lamination is
achieved during manufacture, and that voids do not grow during
storage in extreme environmental conditions.
[0081] The front plane laminate described in the aforementioned
2004/0027327 is preferably prepared (when an encapsulated
electrophoretic medium is used) by first coating and drying a film
of capsules and polymeric binder on a transparent conductor (for
example, indium tin oxide or conductive polymer) carried on the
polymeric film substrate (for which poly(ethylene terephthalate) or
PEN are preferred). The capsules themselves vary in size, but are
preferably 30 .mu.m to 50 .mu.m (85% confidence), more preferably
30 .mu.m to 50 .mu.m (99% confidence), and even more preferably 35
.mu.m to 45 .mu.m (99% confidence).
[0082] The effects of this variation in size of capsules will now
be considered using FIGS. 2 and 3 of the accompanying drawings.
FIG. 2 illustrates schematically a front plane laminate comprising
a transparent front substrate 100 through which the observer views
the display. This front substrate 100 carries a light-transmissive
conductive layer 102, on which is formed an electrophoretic layer
104, illustrated as comprising a plurality of capsules dispersed in
a binder. On the opposed side of the electrophoretic layer 104 from
the conductive layer 102 is provided a release sheet 106. In
practice, a layer of lamination adhesive is normally provided
between the electrophoretic layer 104 and the release sheet 106,
but this lamination adhesive is omitted from FIGS. 2 and 3 for ease
of illustration. FIG. 3 of the accompanying drawings is a schematic
illustration similar to FIG. 2 but showing the front plane laminate
with the release sheet 106 removed, and showing the peak-to-valley
roughness and local surface angle (as discussed below) of the
electrophoretic layer 104.
[0083] It will be apparent that the aforementioned variation in
size of capsules results in an uneven surface of the
electrophoretic (electro-optic) layer 104 facing the release sheet
106 (i.e., the upper surface of the layer 104 as illustrated in
FIGS. 2 and 3). In preferred embodiments, the front substrate is
thick enough to enhance the mechanical ruggedness of the display,
for example by conferring impact resistance. Preferably, the front
substrate is thinner than 20 mil (approx. 0.5 mm). Even more
preferably the front substrate is thinner than 10 mil (approx. 0.25
mm).
[0084] Consideration of the structure of such a front plane
laminate shows that there is an optimum thickness for the release
sheet, which is preferably also formed from poly(ethylene
terephthalate) or PEN, and which may, for reasons explained in the
aforementioned 2004/0027327 bear a conductive layer on its surface
facing the electro-optic medium. If the release sheet is too thin,
it will follow the contours of the electro-optic layer. If these
contours are too closely followed, one finds that, after lamination
of the front plane laminate to the backplane, many air voids exist
between the backplane and the layer of lamination adhesive.
However, if the release sheet is too thick, the release sheet
becomes more costly and it becomes very difficult to maintain the
integrity of the front plane laminate if it is rolled up during
storage. For these reasons, it is generally preferred that the
release sheet be thinner than 15 mil (approx. 0.37 mm), desirably
thinner than 10 mil (approx. 0.25 mm). At present, it is typically
preferred to use a polymeric substrate 7.5 mil (188 .mu.m) thick
and a release sheet 5 mil (approx. 127 .mu.m) thick.
[0085] Since the release sheet is thinner than the front substrate,
the topography of the uneven capsule film is mainly transferred to
the release sheet 106, as shown in FIG. 2. Note that the uneven
topography (known in the imaging industry as "orange peel") is most
apparent on the exposed surface of the release sheet, but is
evident to some degree on the exposed surface of the front
substrate. (As will readily be apparent to those skilled in the
art, FIGS. 2 and 3 show the various layers of the display inverted
with respect to FIG. 1, so that in FIGS. 2 and 3 the exposed
surface of the front substrate, which forms the viewing surface of
the display, is at the bottom of the Figure.)
[0086] There are two requirements for ensuring that voids are not
present in the display cell after lamination of the front plane
laminate to the backplane, namely: (1) there must be sufficient
thickness of adhesive and/or binder to ensure that the interstitial
spaces between capsules are filled with adhesive and/or binder and
some amount of additional adhesive covers the largest diameter
capsules in the layer, and (2) the surface roughness of the front
plane laminate meets certain specifications described below.
[0087] Consideration of FIG. 2, and especially of the largest
capsule shown therein, suggests that there may be situations in
which the largest capsules do not have appreciable amounts of
adhesive and/or binder between them and the release sheet. This
creates a potential defect in the display, where adhesion is
locally reduced and a void is likely to appear under high
temperature stress. Accordingly, it is desirable to select the
adhesive and/or binder material properties and thickness, and to
apply the adhesive and/or binder in such a way that all
interstitial spaces between capsules and all areas above capsules
are coated with adhesive and/or binder. Preferably, the thinnest
region of the adhesive and/or binder should be less than 30 .mu.m,
more preferably less than 20 .mu.m, and desirably less than 10
.mu.m. Of course, it is desirable to keep adhesive and/or binder
thickness as low as possible to reduce voltage drop across the
adhesive and maximize the voltage drop across the electro-optic
medium.
[0088] Once sufficient adhesive and/or binder has been applied to
fill in interstitial sites and cover the largest capsules, it is
important that the surface of the adhesive be sufficiently flat. As
shown in FIG. 3, which is a schematic side elevation similar to
that of FIG. 2 but after removal of the release sheet from the FPL,
this flatness requirement places limits on the surface roughness
(measured as peak-to-valley roughness, .delta.h in FIG. 3) of the
front plane laminate after removal of the release sheet. There are
many ways to specify surface roughness for such a laminate. It is
desirable that the peak-to-valley roughness be less than 15 .mu.m,
preferably less than 10 .mu.m, and most desirably less than 5
.mu.m. In addition, it is also desirable that the local surface
angles (indicated as .theta. in FIG. 3) on the front plane laminate
be less than 15.degree. from the horizontal, preferably less than
10.degree. from the horizontal, and most desirably less than
5.degree. from the horizontal. This restriction on surface angles
helps to ensure that lamination process speeds can be high enough
for mass production.
[0089] FIGS. 4A and 4B of the accompanying drawings show
respectively a photomicrograph of a front plane laminate with long
wavelength peak-to-valley roughness greater than 5 .mu.m, and the
voids that result after thermal lamination of this relatively rough
front plane laminate to a glass backplane. These voids would be
likely to produce unacceptable artifacts in a commercial
electro-optic display.
[0090] It will be appreciated that the optimum adhesive thickness
for adhesive filling of interstitial sites and covering the largest
capsules may vary with capsule size distribution. As an example,
assume the capsule size distribution ranges from 20 .mu.m to 60
.mu.m (i.e., assume 99% of capsules fall within this size range).
After drying the capsules on the front substrate, their thickness
typically drops by about 50% due to coating and drying dynamics, so
the largest dried capsules will be some 30 .mu.m in thickness and
the smallest capsules will be some 10 .mu.m in thickness. Thus, the
peak-to-valley roughness of the resulting capsule layer will be
slightly larger than 20 .mu.m (30 .mu.m for the largest capsule
minus 10 .mu.m for the smallest capsule, plus the distance from the
top of the smallest capsule to the bottom of an interstitial site
next to a small capsule). In such a situation, it is desirable to
apply more than 20 .mu.m, and typically more than 30 .mu.m, of
lamination adhesive to ensure that lamination voids do not exist
after manufacture or after environmental stressing. If the capsule
size distribution were much tighter, say 35 .mu.m to 45 .mu.m (with
99% confidence), then the peak-to-valley roughness would be only
slightly larger then 5 .mu.m and a thinner adhesive film could
safely be applied.
[0091] The following Example 2 is now given to show how the
peak-to-valley roughness of a front plane laminate affects the
formation of voids after lamination of the front plane laminate to
an experimental backplane.
[0092] Front plane laminates and experimental displays were
prepared in substantially the same manner as in Example 1, except
that either no lamination adhesive was applied to the
capsule/binder layer or (nominally) 12, 18, 25, 35 or 45 .mu.m
layers of a custom polyurethane-based lamination adhesive was
applied. The root mean square surface roughness and the
peak-to-valley roughness of the exposed surfaces of the front plane
laminate, after removal of the release sheet, were measured using a
KLA-Tencor Surface Profiler with a 2000 .mu.m (2 mm) scan length.
The laminates were then laminated to the backplane and the
formation of voids immediately after lamination was observed; these
voids are referred to as "t0 voids" in Table 3 below. The
experimental displays thus formed were stored at 90.degree. C. for
15 hours and the voids again observed visually (called "t15" in
Table 3). The results are shown in Table 3 below, in which "Rptv"
denotes peak-to-valley roughness.
TABLE-US-00003 TABLE 3 Adhesive R rms (um) R ptv micron AVG STDEV
AVG STDEV t0 t15 0 2.4 0.4 13.6 2.5 n/a n/a 12 1.2 0.3 8.6 2.2 Many
t0 voids! t0 voids coalescing, still evident but some "healing" 18
0.2 0.2 0.9 1.7 Many t0 voids---fewer t0 voids coalescing, than 12
um level still evident but some "healing" 25 0.3 0.3 1.9 2.0 Few/no
t0 voids Few signs of void growth 35 0.0 0.0 0.2 0.1 No t0 voids No
signs of void growth 45 0.0 0.0 0.2 0.1 No t0 voids No signs of
void growth
[0093] From the data in Table 3, it will be seen that increasing
the thickness of the lamination adhesive reduced the number of
voids. More specifically, the data suggest that, to positively
affect the void growth problem, the adhesive thickness should be
somewhat larger than the maximum observed peak-to-valley height
change of the electrophoretic layer.
[0094] From the foregoing, it will be seen that the buckling
instability of layers in hybrid displays is a complex phenomena
that could jeopardize the success of such products in the
marketplace, especially as specifications on electronic displays
become more and more demanding as competitive technologies such as
liquid crystal displays become more refined. The present invention
provides pathways for mitigating this problem which are well suited
to large scale manufacture.
[0095] Numerous changes and modifications can be made in the
preferred embodiments of the present invention already described
without departing from the spirit and skill of the invention.
Accordingly, the foregoing description is to be construed in an
illustrative and not in a limitative sense.
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